Course of Raku / Advanced / Control flow

gather and take

The gather and take pair is a flexible way to build a list. You wrap some code in a gather block, and every time that code calls take, the value is added to the list that gather produces.

my @result = gather {
    take 1;
    take 2;
    take 3;
};

say @result; # [1 2 3]

The three take calls contribute three values, and gather collects them into the list stored in @result.

The real strength of this pair shows when the take calls are spread through ordinary control flow, such as a loop with a condition. The list then contains only the values you chose to take:

my @evens = gather for 1..10 {
    take $_ if $_ %% 2;
};

say @evens; # [2 4 6 8 10]

Here the loop runs over the numbers from 1 to 10, but only the even ones are taken, so @evens ends up holding 2, 4, 6, 8, 10. (The %% operator tests whether a number divides evenly.)

This separation is what makes gather/take so handy: the surrounding code decides when to produce a value, and gather quietly gathers whatever is taken.

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